In recent years, the use of cellular communication systems having mobile terminals which communicate with a hardwired network, such as a local area network (LAN), has become widespread. Retail stores and warehouses, for example, may use cellular communications systems to track inventory and replenish stock. In manufacturing facilities, such systems are useful for tracking parts, completed products and defects.
A typical cellular communication system includes a number of fixed base stations interconnected by a cable medium to form a hardwired network. The hardwired network is often referred to as a system backbone. Associated with each base station is a geographic cell. A cell is a geographic area in which a base station has sufficient signal strength to transmit data to and receive data from a mobile terminal with an "acceptable" error rate. The error rate for transmitted data is defined as the ratio of the number of transmitted data bits received in error to the total number of bits transmitted. It is economically inefficient to design a communications system with a "zero" error rate. Rather, depending on the requirements of users of the system, an "acceptable" error rate is determined. For example, an "acceptable" error rate may be set at a maximum error correcting rate capability of an error correcting code utilized by the system. The cell associated with a base station is a geographic area in which the station can transmit and receive data with an error rate at or below the "acceptable" error rate. In many instances, the cell of a base station is not circular because physical structures within the cell may partially block data signals emanating from the base station thereby creating "dead spots" which cannot receive transmissions from the station at or below the "acceptable" error rate. The cell size may be decreased by machinery located in the vicinity of the base station which generates excessive noise levels that degrade a signal transmitted by the base station. Undesirable signals that interfere with the transmission and reception of a transmitted signal are collectively referred to as noise signals. A useful quantitative measure of relative noise in a communication system is the signal-to-noise ratio (SNR). The SNR is the ratio of the amplitude of a desired signal at any point to the amplitude of noise signals at that same point.
Mobile terminals, such as hand-held units or portable units mounted in a vehicle, may communicate with other devices on the network via wireless communication links through one or more of the base stations. Generally, when a mobile terminal is powered up, it "registers" with a base station which provides the strongest SNR. However, as the location of this mobile terminal changes, it may "de-register" with the first base station and register with another base station which provides a stronger SNR.
When designing a cellular communication system for a region, an appropriate number of base stations must be selected and their locations determined to assure cell coverage for the region. Each additional base station increases the cost of the communication system by the incremental cost of the base station and the cable medium to link the additional base station to the backbone. Because the conditions of the region may result in "dead spots" in a base station's cell and since the cost of a base station increases with the transmission power of the station, it is generally not feasible to provide one high-powered base station to service the entire region. In addition, the limited power supply of a wireless mobile terminal provides for a limited communication range. Also, FCC regulations limit the transmission power of transmitters. Thus, the design of a cellular communication system involves consideration of a complex set of variables including the conditions of the region, transmitted power and cost of base stations, time and spatial patterns of system usage, sources of noise, "acceptable" error rate or rates, etc.
A spread spectrum (SS) communication system is one in which the transmitted frequency spectrum or bandwidth is much wider than absolutely necessary. Wideband frequency modulation (FM) is an example of an analog SS communication system. With regard to a digital SS communication system, the transmission bandwidth required by the baseband modulation of a digital signal is expanded to a wider bandwidth by using a much faster switching rate than used to represent the original bit period. Operationally, prior to transmission, each original data bit to be transmitted is converted or coded to a sequence of "sub bits" often referred to as "chips" (having values of zero or one) in accordance with a conversion algorithm. The coding algorithm is usually termed a spreading function. Depending on the spreading function, the original data bit may be converted to a sequence of five, ten, or more chips. The rate of transmission of chips by a transmitter is defined as the "chipping rate".
A SS communication system transmits chips using a wider signal bandwidth (broadband signal) and a lower signal amplitude then the corresponding original data would have been transmitted at baseband. At the receiver, a despreading function and a demodulator are employed to convert or decode the transmitted chip code sequence back to the original data on baseband. The receiver, of course, must receive the broadband signal at the transmitter chipping rate.
An advantage of a SS communication system is that the representation and communication of an original data bit as a sequence of chips over a wide bandwidth in lieu of transmitting the original data bit over a narrow bandwidth generally results in a lower error rate at the receiver. This is especially true in transmission environments characterized by noise having high amplitude and short duration, i.e., "spike" noise. The probability of a receiver extracting and correctly interpreting a data bit represented by a transmitted sequence of chips interspersed with random, uncorrelated noise spikes is greater than the probability of the receiver extracting and correctly interpreting a transmission of single bits interspersed with such random noise spikes.
In essence, a SS communication system utilizes increased bandwidth and a coding scheme to reduce error rate vis-a-vis a "conventional" baseband system. The reduction in error rate results in an improved output SNR at the receiver. For any communication system, the difference between output SNR and input SNR is defined as the processing gain of the system. In a SS communication system, the processing gain of the system is the ratio of the transmission code rate to the original information bit rate. For example, assume that the SS coding scheme utilizes a sequence of ten chips to represent one original data bit. If the ten chips are transmitted at a chipping rate such that their collective duration is equal to a single bit period at baseband, then the processing gain of the SS system is approximately equal to ten. Communication range is determined by a fully processed SNR at a receiver. The fully processed S/N is the processing gain associated with SS communication techniques combined with the received signal strength.
The coding scheme of a SS digital communication system utilizes a pseduo-random binary sequence (PRSB). One type of a digital SS communication system is known as a direct sequence spread spectrum (DSSS) system. In a DSSS system, coding is achieved by converting each original data bit (zero or one) to a predetermined repetitive pseudo noise (PN) code. A type of PN code is illustrated in FIG. 1. For this example, the digital data signal 110 is made up of a binary "1" bit and a "0" bit. A PN code 120 is comprised of a sequence of ten sub bits or chips, namely, "1", "0", "1", "1", "0", "1", "1", "1", "0", "1".
The digital data signal 110 is coded or spread by modulo 2 multiplying (e.g., via an "EXCLUSIVE NOR" (XNOR) function) of the digital data signal 110 with the PN code 120. If the data bit is a "1", then the resulting spread data signal (PN coded signal) in digital form corresponds to the PN code 120. However, if the data bit to be coded is a "0", then the spread data signal in digital form will correspond to a code 130. As can be seen, the code 130 is the inverse of PN code 120. That is, the PN code and its inverse are used to represent data bits "1" and "0" respectively.
A PN coded length refers to a length of the coded sequence (the number of chips) for each original data bit. As noted above, the PN code length affects the processing gain. A longer PN code yields a higher processing gain which results in an increased communication range. The PN code chipping rate refers to the rate at which the chips are transmitted by a transmitter system. A receiver system must receive, demodulate and despread the PN coded chip sequence at the chipping rate utilized by the transmitter system. At a higher chipping rate, the receiver system is allotted a smaller amount of time to receive, demodulate and despread the chip sequence. As the chipping rate increases so to will the error rate. Thus, a higher chipping rate effectively reduces communication range. Conversely, decreasing the chipping rate increases communication range.
The spreading of a digital data signal by the PN code does not effect to overall signal strength (or power) the data being transmitted or received. However, by spreading a signal, the amplitude at any one point will be less then the original (non-spread) signal.
It should be appreciated that increasing the PN code length or decreasing the chipping rate to achieve a longer communication range will result in a slower data transmission rate. Correspondingly, decreasing the PN code length or increasing the chipping rate will increase data transmission rate at a price of reducing communication range.
FIG. 1A schematically illustrates a transmitter system or assembly 100 of a DSSS system. Original data bits 101 are input to the transmitter system 100. The transmitter system includes a modulator 102, a spreading function 104 and a transmit filter 106. The spreading function 104 converts the modulated original data bits 101 into a PN coded chip sequence 108, also referred to as spread data. The modulator 102 modulates the data onto a carrier using, for example, a binary phase shift keying (BPSK) modulation technique. The BPSK modulation technique involves transmitting the carrier in-phase with the oscillations of an oscillator or 180 degrees out-of-phase with the oscillator depending on whether the transmitted bit is a "0" or a "1".
FIG. 1A also illustrates a receiver system or assembly, shown generally at 150. The receiver system 150 includes a receive filter 152, a despreading function 154, a bandpass filter 156 and a demodulator 158. The received PN coded data 108 is decoded by a PN code despreading function 154. The decoded data is then filtered and demodulated by the filter 156 and the demodulator 158 respectively to reconstitute the original data bits 101. To receive the transmitted spread data, the receiver system 150 must be tuned to the same predetermined carrier frequency and be set to demodulate a BPSK signal using the same predetermined PN code.
To receive a SS transmission signal, the receiver system must be tuned to the same frequency as the transmitter assembly to receive the data. Furthermore, the receiver assembly must use a demodulation technique which corresponds to the particular modulation technique used by the transmitter assembly (i.e. same PN code length, same chipping rate, BPSK). Because mobile terminals communicate with a common base station, each device in the cellular network must use the same carrier frequency and modulation technique.
A drawback associated with current cellular communication systems is that a PN code length must be selected to provide performance based on average communication range and average noise conditions. The data rate/range tradeoff leads to a cell size/throughput tradeoff in the communication system. The rate that each transmission occurs at will limit the size of each cell. Thus, it would be desirable to have a cellular communication system wherein processing gain and/or chipping rate could be dynamically modified for each transmission based on distance between the transmitter and receiver and noise conditions such that an optimal data transmission rate for that transmission could be achieved thereby enhancing system performance.